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WATERMARK clause

  • Artikkel

Applies to: check marked yes Databricks SQL check marked yes Databricks Runtime 12.0 and above

Adds a watermark to a relation in a select statement. The WATERMARK clause only applies to queries on stateful streaming data, which include stream-stream joins and aggregation.

Syntax

from_item
{ table_name [ TABLESAMPLE clause ] [ watermark_clause ] [ table_alias ] |
  JOIN clause |
  [ LATERAL ] table_valued_function [ table_alias ] |
  VALUE clause |
  [ LATERAL ] ( query ) [ TABLESAMPLE clause ] [ watermark_clause ] [ table_alias ] }

watermark_clause
  WATERMARK named_expression DELAY OF interval

Parameters

  • named_expression

    An expression that provides a value of type timestamp. The expression must be either a reference to the existing column, or a deterministic transformation against existing column(s). The expression adds a column of timestamp type which is used to track the watermark. The added column is available to query.

  • interval_clause

    An interval literal that defines the delay threshold of the watermark. Must be a positive value less than a month.

Examples

-- Creating a streaming table performing time window row count, with defining watermark from existing column
> CREATE OR REFRESH STREAMING TABLE window_agg_1
  AS SELECT window(ts, '10 seconds') as w, count(*) as CNT
  FROM
  STREAM stream_source WATERMARK ts DELAY OF INTERVAL 10 SECONDS AS stream
  GROUP BY window(ts, '10 seconds');

-- Creating a streaming table performing time window row count, with deriving a new timestamp column to define watermark
> CREATE OR REFRESH STREAMING TABLE window_agg_2
  AS SELECT window(ts, '10 seconds') as w, count(*) as CNT
  FROM
  STREAM stream_source WATERMARK to_timestamp(ts_str) AS ts DELAY OF INTERVAL 10 SECONDS AS stream
  GROUP BY window(ts, '10 seconds');